In the production of paper in a papermaking machine a slurry of fibers suspended in a water is discharged onto a wire of a mesh-like construction. The water is drained off from the fiber slurry through the openings formed in the wire. The fibers are formed into a paper sheet on the upper surface of the cloth. Since the water through-flow takes place at the points where the thread material of the cloth does not prevent water drainage it is of the greatest importance that these through-flow points are evenly distributed across the entire surface of the cloth. The permeability of the forming fabric must be of a certain magnitude while at the same time the fabric surface must be a very fine-mesh weave in order to prevent marking on the paper sheet and fiber losses. However, fine-mesh cloths made from thin threads have reduced resistance to wear and abrasion and possess impaired stability and as a result their serviceable life is highly shortened. Up to the beginning of the 1960s only single-layer fabrics or wires of metal were used for the formation of paper sheets. To some extent the metal wire were replaced by single-layer fabrics of synthetic fiber threads, known as plastic cloths, which although being more resistant to wear and abrasion were also less stable than the metal ones. On account of their poor stability single-layer plastic cloths cannot be used in large high-speed papermaking machines. Not until the advent of the double-layer forming fabrics as they are called could synthetic fiber materials be utilized to any noticeable extent in these large high-speed machines. In this type of machine the forming fabric is exposed to considerable tensile stress which the fabric has to be able to take without stretching lengthwise or contracting crosswise to such an extent than the fabric loses its ability to serve its intended purpose. By the expression "double-layer forming fabric" is for the purposes herein to be understood forming fabrics consisting of two layers of synthetic weft threads and of synthetic warp threads interconnecting said two weft thread layers.
The next step of development concerned a forming fabric comprising two complete interwoven weaves, each one having its individual set of warp threads and weft threads. The part of fabric closest to the material to be formed as a rule consists of fine threads forming a fine-mesh construction whereas the bottom weave consists of a coarser and more wear-resistant threads forming a coarser-mesh construction.
The double-layer forming fabric comprising two layers of weft threads and the double weave comprising two complete weaves have proved to be supplementary. The first type of fabric is preferably used in the majority of large papermaking machines whereas the last-mentioned fabric is most suitable for use in positions where the demands on the quality and properties of the paper surface are extremely strict, such as is the case as regards high-quality paper and magazine paper. However, from a technical and practical point of view it is a serious drawback to have to use two different systems of warps and in addition each system of this kind must differ as regards the dimensions of the warp threads used and the density of the warp threads.
This drawback has been removed through the subject invention which is characterized in that the warp threads of the second system of warp threads which in position of use of the fabric faces the dewatering elements of the machine are arranged in groups comprising at least two adjacent warp threads, which two threads weave in an identical manner and in essentially parallel with each other when weaving with the weft threads of the second system of weft threads.
The arrangement in accordance with the subject invention makes it possible to produce double-layer type of fabrics as well as double-weave types of fabrics, using the same warp having equal thread dimensions and thread densities. For instance, a warp comprising threads of a dimension of 0.22 mm may be set up in a reed with a thread density of 52 threads per cm. Initially is woven a double-layer forming fabric in which all warp threads are used to interconnect the two layers of weft thread. The warp fill factor, that is, the area of the fabric surface which theoretically is covered by warp threads, in this case is 0.20.times.5.2=1.04, which is completely normal in these types of weaves. While retaining the same warp set-up it is then possible to weave a double-weave construction. Half the number of the warp threads, that is 26 per cm, are in this case used to interweave with a first system of weft threads so as to form a first weave which in position of use of the final fabric faces the material to be formed. The warp fill factor for this weave is 0.20.times.2.6=0.52, which is likewise normal in the case of single-layer constructions. The other half of the warp threads are used to interweave with a second system of weft threads so as to form a second weave which in position of use of the final fabric faces the dewatering elements of the machine. To achieve this, the warp threads are arranged in groups comprising two threads each, which two threads weave in an identical manner and in essentially parallel with each other when weaving with the weft threads of the second system of weft threads. These double warp threads, which thus are 13 double or paired threads per centimeter, replace the coarser type of warp threads which have hitherto been used, the number of these coarser threads usually being half that of the threads of the first weave.
In addition to providing the technical and practical advantages outlined above the structure in accordance with the teachings of the subject invention also provides the same advantages as those found in thinner fabrics as compared with corresponding forming fabrics comprising coarser threads in the bottom warp. It has been found that the forming fabric in accordance with the subject invention is also more stable than the prior-art fabrics.